Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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JET-BASED MIXER SETTLER
BACKGROUND OF THE INVENTION
[0001] This invention relates to a jet-based mixer settler system.
[0002] Various mixer settler arrangements have been proposed in the prior art.
[0003] US3126258 discloses the simultaneous use of separate mixer and settler
compartments for each stage in a plurality of stages in a complex system. Jet
mixing
occurs as liquids are withdrawn from a mixing compartment and are injected
back
into the mixing compartment through several jets. Although internal moving
parts are
not required the jet mixing does not provide functionality which is
meaningfully
different from that achieved through mechanical mixing. A settler section
operates
continuously at a rate which is determined by the requirement of settling in
each
stage at the full rate of each counter-current stream. The throughput rate is
therefore
not controllable over a wide range and can be subjected to a condition known
as
"flooding", when settling is incomplete and mixed liquids are transferred
instead of
separated liquids. Transfers of liquids take place simultaneously through
interconnecting pipes or ducts and depend on gravity action in combination
with fluid
density differences and the relative elevation of the successive stages. As
these
elevations are fixed the possibility of inducing widely different flow ratios
is limited.
[0004] US2646346; US2754179; US2980514; US3089756 and US320688 disclose
systems which have separate mixer and settler compartments with mechanical
mixing and which generally exhibit characteristics similar to those identified
herein for
US3126258.
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[0005] US3549332 discloses three distinct timed sub-operations for mixing and
for
the transfers of light and heavy fluids in opposite directions. Inter stage
ducts
connect the bottom of one stage to the top of another stage so as to
facilitate
different transfers forwards and backwards. The ducts are not used for mixing.
There is no suggestion in this patent of the possibility of using multiple
physical
stages per operational stage. Liquid transfers always involve one liquid
entering
another liquid. Thus a transfer of a liquid volume greater than the normal
operating
volume of that liquid, in a stage, requires the liquid to settle or rise
quickly through
the other liquid. This can possibly cause adverse mixing and entrainment. This
is an
important factor in large scale systems for adverse mixing can be problematic
as flow
cross-sectional areas in the stages scale up only in proportion to flow rates
raised to
2/3, for similar residence times. In this citation mixing is done by
mechanical stirring
and there is no suggestion of eliminating the mechanical means of mixing, by
the use
of transfer flows instead. The technique described in this citation is not
applicable to
liquid/solid operations because the transfer of any fluid entails a transfer
from deep
within a settled bed of solids where blockages can easily occur.
[0006] In this specification the word "light" is used, for the sake of
convenience, to
designate a fluid which is less dense than another fluid which, in turn, is
referred to
as "a dense fluid".
[0007] Figure 1 of the accompanying drawings illustrates apparatus, 10
referred to
as a mixer settler column in the specification of South African Patent
Application No.
2012/03142, which allows for mixing, settling, and counter-current transfers
of fluid to
be done cyclically in a plurality of distinct modes of operation. These
operations are
carried out in different time periods in the same physical regions instead of
using
different physical regions for the various operations.
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[0008] The apparatus includes a column which is vertically orientated. A
single
column, with standard plates, can be used for any of a number of processes and
can
be used over a wide range of operating conditions. Modes of usage include
liquid-to-
liquid contacting operation, counter-current decantation, counter-current ion
exchange, counter-current leaching and thickening.
[0009] US2629654 discloses a structure which includes orifices having
different
forms which promotes mixing of a lighter phase with a heavier phase.
Differently
shaped and positioned orifices are disclosed in this citation. These orifices
are
positioned in a plate between adjacent vessels and effectively place the
liquids in
adjacent vessels in direct communication with each other. Although the
arrangement
does allow for intimate contacting of two counter-flowing bodies of partially
immiscible fluids the orifices do not maximise turbulence and hence do not
maximise
mixing of such fluids.
[0010] An object of the present invention is to provide apparatus for multi-
unit
counter-current mixer settler operations in which the units are arranged
horizontally
and wherein use is made of a transfer duct which promotes mixing of a light
fluid with
a heavier fluid to a maximum extent. Such operations also include liquid-to-
liquid
contacting operation, counter-current decantation, counter-current ion
exchange,
counter-current leaching and thickening.
[0011] The horizontal arrangement of units can provide several advantages
depending on practical needs and constraints. In some instances, headroom for
support structure for a horizontal set of units (or a horizontally oriented
apparatus)
can usually be less complex and costly than for a vertical column. A
horizontally
arranged set of units can often be more easily maintained, by work at ground
level,
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and by the draining, cleaning and repairing of individual units. In instances
where
many units are needed in a process, a high or a long series of units can be
avoided
by using a vertical column that has vertically stacked layers, each with a
number of
horizontally arranged units.
SUMMARY OF THE INVENTION
[0012] The invention provides a mixer settler which includes, at least, first
and
second units which are horizontally disposed relative to each other, each unit
including a respective enclosed vessel, and a transfer duct which includes a
first end
which is exposed to an upper region of the first unit and a second end which
is
exposed to an upper region of the second unit, which connects the upper region
of
the first unit to the upper region of the second unit and which is shaped so
that fluid
passing from the first unit to the second unit increases in velocity. This
velocity
increase takes place along the length of the transfer duct. Additionally the
transfer
duct is orientated (its discharge end is angled) so that the emitted jet
traverses a path
of maximum length inside the second unit. This promotes turbulence and
effective
mixing of the light and heavy phases.
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[0013] The mixer settler may be connected by fluid ports to an external source
which applies pressure to the fluids in the units.
[0014] The respective vessel in each unit may be cylindrical. The units may be
positioned with a common horizontal axis. The units may abut each other and
have
vertical partitions between them. Thus adjacent units may be arranged with
common
or closely spaced walls. Alternatively the units may be horizontally spaced
apart.
[0015] Each unit has a fluid-containing volume below the height of the ducts
which
tends to trap dense fluid and which tends to prevent the trapped dense fluid
from
flowing towards an adjacent unit.
[0016] The transfer duct may be tapered i.e. it may reduce in cross-sectional
area in
a direction from the first unit to the second unit.
[0017] When fluid flows at a relatively high flow rate from the first end to
the second
end of the duct, the second end may perform the function of a jet nozzle to
eject fluid
into the second unit, preferably in a direction towards a location inside an
interior of
the second unit which is displaced by a maximum distance from the nozzle (the
second end of the duct). In this case, the jet of ejected fluid causes
turbulent mixing
of the fluid in the second unit.
[0018] When fluid flows at a low to moderate flow rate from the second end to
the
first end of the duct, the first end may perform the function of ejecting
fluid into the
first unit at a relatively low velocity in a way that tends not to disturb any
dense fluid
within a containing volume of the first unit. In this case, light fluid from
above the bed
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of dense fluid in the second unit is transferred to mix with, or displace,
light fluid
above the bed of dense fluid in the first unit.
[0018] The transfer duct may be located externally to the first and second
units.
Alternatively it may be located within the first and second units. In each
case the duct
5 may protrude some distance into one or both of the units.
[0019] According to requirement a third unit may be connected to the second
unit in
a manner which is similar to that which has been described with respect to the
second and the first units, a fourth unit may be connected to the third unit
in a similar
manner, and so on.
[0020] Other arrangements are possible. For example the units may be disposed
in
a column configuration which includes a number of layers of units wherein the
layers
are vertically stacked.
(0021] The individual units may be of varying lengths and shapes but their
volumes
and duct cross-sectional areas are typically substantially the same in size.
According
to requirement each unit may include a drainage port or valve for inspection
and
maintenance, as required.
[0022] The term "mixing feed end" is used to designate an end of the series of
units
of the apparatus such that if a fluid is introduced at that end, it tends to
cause fluid to
flow within the inter-connecting ducts of the apparatus from the first ends to
the
second ends of the respective ducts. Conversely, the term "transfer feed end"
is used
to designate an end of the series of units such that if a fluid is introduced
at that end,
it tends to cause fluid to flow within the inter-connecting ducts of the
apparatus from
the second ends to the first ends of the respective ducts.
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[0023] Mixing may be carried out for a predetermined period of an operational
cycle
("a mixing sub-cycle") by introducing a fluid with a suitably high flow rate
into the
apparatus at or near its mixing feed end thereby causing a flow of fluid
towards the
transfer feed end and causing the mixing of dense and light fluids in each
unit within
the apparatus. The volume of dense fluid transferred in such a period may be
the
volume of dense fluid required to be processed by the apparatus for a complete
operational cycle.
[0024] Settling may be carried out for a predetermined period of the
operational
cycle after mixing. During this period, fluid flows into and out of the ends
of the
apparatus are stopped.
[0025] The transfer of light fluid may be carried out for a predetermined
period of the
operational cycle, after settling, by introducing a fluid with a suitably low
to moderate
flow rate into the apparatus at or near its transfer feed end thereby causing
a flow of
fluid towards the mixing feed end and the transfer of light fluid only from
unit to unit
within the apparatus. The volume of fluid transferred in such a period may
comprise
a volume sufficient to return a similar volume of light fluid as was displaced
during
the preceding mixing sub-cycle and a further volume of light fluid to produce
an
overall light fluid movement as might be required to be processed by the
apparatus
for a complete operational cycle.
[0026] The counter-current flow of light and dense fluids requires an
arrangement at
the transfer feed end of the apparatus to separate a dense fluid product from
the light
fluid in the associated unit. This separation may be done outside the units of
the
apparatus or in an end unit of the apparatus. In the latter case, the transfer
duct of
the end unit at the transfer feed end should preferably not be tapered into a
jet, and
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there may be two fluid ports at that end, with an upper two-way port for the
light
liquid, at a height similar to that of the transfer ducts, and a lower outlet
port for the
dense liquid, at a height below the top of the contained volume of the unit.
[0027] In the case of the mixing feed end, only light fluid exits from the
mixing feed
port, during the transfer period, and a feed of dense fluid and some returned
light
fluid is induced by external means to enter the same port during the mixing
period.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The invention is further described by way of examples with reference to
the
accompanying drawings in which:
Figure 1 schematically illustrates from one side a jet-based mixer settler
which
includes a number of horizontally arranged units with external inter-unit
ducts;
Figure 2 shows a system which is similar to that shown in Figure 1 but with
internal
inter-unit ducts;
Figure 3 shows an arrangement which is substantially the same as the Figure 2
system but with an end unit at the transfer feed end having two fluid ports
and an
inter-unit duct that is not tapered to a jet nozzle;
Figure 4 is a side view of an apparatus which includes stacked layers of
horizontally
arranged units; and
Figure 5 is a plan view in section of the Figure 4 mixer settler apparatus.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] Figure 1 shows in cross-section and from one side a mixer settler
apparatus
10 according to the invention. The apparatus includes an array of horizontally
arranged units 12, 14 and 16. Three units are shown. This is not limiting and
the
number of units in the array can be varied. Each unit comprises a respective
closed
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vessel. The vessels are arranged on a common horizontal base or axis 20. In
this
example the vessels are close to each other, essentially abutting one another,
or are
integrally formed and have vertical partitions 22 between adjacent vessels.
[0030] Each vessel has a respective drainage valve 26. Ports, not shown, for
inspection and maintenance can be installed at the top of each unit if
necessary.
[0031] The first unit 12 has at least one fluid port 30 and the last unit in
the array has
at least one fluid port 32. Respective transfer ducts 40 and 42 are positioned
between adjacent vessels. The duct 40 has an inlet 40A which is exposed to an
upper region 12A of the unit 12. The duct has a tapered shape and reduces in
cross-
sectional area towards its outlet 12B which is in an upper region 14A of the
unit 14.
The duct outlet is not aimed vertically downwards but, instead, is angled so
that fluid
emitted by the duct tends to flow along a maximum length inside the unit 14 ¨
this is
indicated by a dotted line 44 which extends from the outlet 12B towards a
diagonally
positioned lower end 14B of the unit. The intention in this respect is to
induce
maximum turbulence in the fluid inside the vessel. The duct 42 is similarly
orientated.
[0032] The apparatus 10 has a mixing feed end 46 and a transfer feed end 48.
The
fluid port 32, in the last unit 16, is a two-way port i.e. a port for the
light fluid and an
exit port for the dense fluid.
[0033] The transfer ducts 40 and 42 are external to the units.
[0034] A controlled pressure source 50 is connected to the port 30.
[0035] Figure 2 shows a mixer settler system 10A according to the invention
which
is substantially the same as what has been shown in Figure 1 with an array of
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horizontally arranged units 52, 54 and 56. However transfer ducts 60 and 62
between adjacent units are positioned inside the respective units. Similarly,
a fluid
port 64 has a portion located inside the unit 52 and a bidirectional fluid
port 66 has a
portion located inside the unit 56.
[0036] In both Figures adjacent units are arranged with common or closely
spaced
walls. Thus the units can be integral with one another or distinct vessels.
The units
may, however, be spaced apart and may be interconnected by means of ducts of
the
required tapering form. The units need not be on a straight line but instead
could be
arranged, viewed in plan, along a curve or in combinations of interconnected
straight
lines and curves.
[0037] Figure 3 shows an arrangement which is substantially the same as what
is
shown in Figure 2 and for this reason the Figure 3 construction is not
described in
detail. Like components bear like reference numerals. The main differences
between the Figure 2 and Figure 3 arrangements are that, in the latter case, a
duct
62A, between adjacent units 54 and 56, is not tapered to a jet nozzle, and
there is an
exit port 78 for the dense fluid which is positioned close to a base of the
unit 56.
Instead of requiring an external means for the separation of light and dense
fluids,
the arrangement of Figure 3 facilitates the separation of light and dense
fluids within
the unit at the transfer feed end of the apparatus, through the absence of
turbulent
mixing by a jet nozzle in the end unit,.
[0038] Figures 4 and 5 are side and plan views respectively of a mixer settler
apparatus 80 which includes a cylindrical housing 82. Inside the housing are
six
stacked layers 84 to 94 of units and, within each layer, four respective
horizontally
arranged units 84A, 84B, 84C and 84D, etc.
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[0039] The individual units are of varying lengths and shapes but their
volumes and
duct cross-sectional areas are typically of the same order of magnitude. The
end
units of each horizontal layer are connected to units above or below by means
of a
respective duct P, Q, R etc. that can include a portion of the wall of the
apparatus as
5 part of the duct wall.
[0040] The ducts P, Q, R etc. are internal. Similarly, transfer ducts between
adjacent units, marked, by way of example only, X, Y, Z for the layer 90 are
internal
and are configured in the manner which has been described hereinbefore in
connection with Figures 1 and 2.
10 [0041] The mixer settler apparatus 80 has a fluid port 100 connected to
the unit 94A
in the uppermost layer 94 and a fluid port 102 connected to the unit 84A in
the
lowermost layer 84.
[0042] In a mixer settler apparatus according to the invention each unit, as
noted,
may comprise a sealed vessel. Thus the movement of fluids between adjacent
units
can be made to depend solely on an externally applied pressure and is not
necessarily dependent on gravity.
[0043] The apparatus is operated cyclically, with periods of mixing, settling
and
transfer, in each operational cycle.
[0044] Mixing within each unit is induced by turbulent eddies created by a
high flow
rate of fluid, entering the mixing feed end of the apparatus, and produced by
the
pressure source 50 and a simultaneous high flow rate of fluid from unit to
unit
towards the transfer feed end. Each duct directs a downward jet of fluid
through the
corresponding downstream adjacent unit and creates turbulence in the fluid in
that
unit. During the mixing phase, dense and light fluids are transferred, from
unit to
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unit, in the direction from the mixing feed end to the transfer feed end of
the
apparatus. The dense fluid does not flow between units at any time other than
during
the mixing phase.
[0045] Settling of the dense fluid in each unit occurs during a settling
period of the
operational cycle, when the flows to and from the apparatus are all made to be
zero.
[0046] Transfer of light fluid, from unit to unit, in the direction from the
transfer feed
end to the mixing feed end of the apparatus occurs during a transfer period of
the
operational cycle, such that the movement of light fluid during mixing is
substantially
reversed and such that a further volume of light fluid is transferred to
result in a net
flow of light fluid over the operational cycle towards the mixing feed end.
[0047] The mixer settler system of the invention thus has a single compartment
per
stage in which mixing and settling are made to occur, at different times in an
operational cycle. Mixing is effected by means of a fluid jet and by flow in
one
direction from one stage to another. Thus mixing requires only one driving
pressure
that is applied externally at one end of the apparatus.
[0048] In the system of the invention settling does not occur continuously but
is a
batch process which is part of a full operational cycle. Provision is thus
made for
operational stages to comprise multiple physical stages (i.e. not necessarily
one
stage only) thereby allowing adjustment of the settling area per operational
stage to
be effected dynamically in a manner which takes into account the number of
operational stages over the column. Additionally, transfer flows can be
effected by
externally applied fluid pressures over different time periods to allow for
significant
flexibility that is not dependent on gravity.
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[0049] The ducts which connect the upper ends of adjacent stages facilitate
mixing
and fluid transfer. The ducts connect only light liquids in adjacent stages
and
transfer can be done by relatively high flows and in large volumes without
mixing.
This is important in large scale systems. Mixing is effected, as noted, by
means of
jets which are induced within the process by a single external pressure
source. Thus
the transfer of fluid between at least one pair of adjacent units is used to
induce
mixing and in this way the need for a separate mixing method is eliminated.
The
system of the invention is, furthermore, applicable to liquid/solid operations
and
allows for the effective handling of settled solid beds.
